Here are some things to check:
Are you your login details correct? Check them on your financial institution's website, then go back into the app and try reconnecting your account.
Are there outstanding actions to take with your bank account? Sometimes Plaid is unable to connect with a financial institution because the user has an outstanding required action, such as the user needs to set up a new security requirement or agree to additional terms of service. You can resolve these issues by logging into your online banking portal and completing the required actions.
Is your financial institution having technical difficulties? Sometimes your bank's servers might be down for maintenance. Check your institution's availability by logging into your online banking portal and then reconnecting to your banking information through Plaid.